Photo 51 is the nickname given to an X-ray diffraction image of DNA taken by Raymond Gosling in May 1952, under the direction of Rosalind Franklin at King's College London in Sir John Randall's group. It was critical evidence in identifying the structure of DNA.
James D. Watson was shown the photo by Maurice Wilkins, who had been given it by Raymond Gosling; along with Francis Crick, he used Photo 51 to develop the first chemical model of DNA, for which the three men jointly won the 1962 Nobel Prize in Physiology or Medicine. As the Nobel prize is not awarded posthumously, Franklin, who had died in 1958, was not eligible for nomination.
The photograph provided key information that was essential for developing a model of B-form (hydrated) DNA. In particular, it could be determined from the diffraction pattern, and was openly discussed by Franklin in lectures attended by Watson and in reports accessible to Watson and Crick, that DNA (1) was helical, (2) was likely a double helix with antiparallel strands, and, (3) had the phosphate backbone on the outside (thus the bases of DNA, which are the "code" for inheritance, were on the inside of the helix). Calculations from the photograph also provided crucial parameters for the size of the helix and its structure, all of which were critical for the molecular modeling undertaken by Watson and Crick.
Photo 51 was, therefore, the critical data that led to the model and confirmation of the postulated double helical structure of DNA, published during 1953 in a series of five articles in the journal Nature. Franklin and Raymond Gosling's own publication in the same issue of Nature was the first publication of this more clarified X-ray image of DNA.
As historians of science have re-examined the period during which this image was obtained, considerable controversy has arisen over both the significance of the contribution of this image to the work of Watson and Crick, as well as the methods by which they obtained the image. Franklin was hired independently of Maurice Wilkins, who, nonetheless, showed Photo 51 to Watson and Crick, without her knowledge. Whether Franklin would have deduced the structure of DNA on her own, from her own data, had Watson and Crick not obtained her image, is a hotly debated topic, made more controversial by the negative caricature of Franklin presented in Watson's history of the research on DNA structure, The Double Helix. Watson later admitted his distortion of Franklin in his book, noting in a preface to a later edition: "Since my initial impressions about [Franklin], both scientific and personal (as recorded in the early pages of this book) were often wrong I want to say something here about her achievements."
Cultural references 
- A play entitled Photograph 51 by Anna Ziegler focuses on the role of x-ray crystallographer Rosalind Franklin in the discovery of the structure of DNA. This play won the 3rd STAGE International Script Competition, and a film version is being produced by Darren Aronofsky, Rachel Weisz and Ari Handel.
- "Due credit". Nature 496, 270 (18 April 2013)
- DNA: the King's story
- Secret of Photo 51. Nova
- The gene: a historical perspective p.85. Greenwood Publishing Group, 2007
- Watson JD, Crick FHC (1953). "A Structure for Deoxyribose Nucleic Acid". Nature 171: 737–738. Full text PDF
- My aunt, the DNA pioneer BBC News. Retrieved May 18, 2011
- Maddox, Brenda (2002). Rosalind Franklin: The Dark Lady of DNA. HarperCollins. ISBN 0-06-018407-8.
- Watson JD, Crick FHC (1953). "A Structure for Deoxyribose Nucleic Acid". Nature 171: 737–738.
- Maddox, pg. 199
- "The instant I saw the picture my mouth fell open and my pulse began to race." -- James D. Watson (1968), The Double Helix, page 167. New York: Atheneum, Library of Congress card number 68-16217. Page 168 shows the X-shaped pattern of the B-form of DNA, clearly indicating crucial details of its helical structure to Watson and Crick.
- Double Helix: 50 Years of DNA. Nature archives. Nature Publishing Group
- Franklin R, Gosling RG (1953) "Molecular Configuration in Sodium Thymonucleate". Nature 171: 740–741. Full text PDF
- Maddox, Brenda (2002). Rosalind Franklin: the dark lady of DNA. HarperCollins. ISBN 0-393-32044-8.
- Max Perutz and the Secret of Life. Published in the UK by Chatto & Windus (ISBN 0-7011-7695-4), and in the USA by the Cold Spring Harbor Laboratory Press.
- Wilkins, Wilkins, M., The Third Man of the Double Helix, an autobiography (2003) Oxford University Press, Oxford
- James D. Watson, The Double Helix: A Personal Account of the Discovery of the Structure of DNA (1968), Atheneum, 1980, ISBN 0-689-70602-2
- Sayre, Anne. Rosalind Franklin and DNA (1975), New York: W.W. Norton and Company, ISBN 0-393-32044-8
- James D. Watson, The Double Helix: A Personal Account of the Discovery of the Structure of DNA (1980 Norton Critical Edition), editor Gunther Stent, W.W. Norton, ISBN 0-393-95075-1.
- Kuchment, Anna (January 2011), "For Whom the Nobel Tolls: An evening out with James Watson and colleagues", Scientific American (Nature America) 304 (1): 27, retrieved 27 March 2011
- Gordon Cox (12 April 2011). "Six get Tribeca Sloan grants". Variety. Retrieved 25 October 2011.